(1) Field of the Invention
This invention relates to novel aromatic amine resins, processes for producing the same and thermosetting resin compositions containing the aromatic amine resins.
(2) Description of the Related Art
It has been known for many years that aromatic amine resins are condensation products of aromatic amines and formaldehyde. For example, an aniline-formaldehyde resin represented by the following general formula (f): ##STR2## has been produced [K. Frey, Herbetica Chemie Acta, 18, 481 (1935)].
However, it is difficult to introduce maleimide or isocyanate groups into conventional aromatic amine resins such as an aniline-formaldehyde resin represented by the general formula (f). Such amine resins are hence not suitable as raw materials for maleimide resins or isocyanate resins. Conventional aromatic amine resins have therefore been used widely as curing agents. Today, increasingly sophisticated application and more rigorous expectation in performance and consistency of performance require that aromatic amine resins be suitable for use in matrix resins of heatresistant composite materials, high-temperatureresistant bonding agents and the like. These demands can no longer always be met adequately by conventional aromatic amine resins.
Heat-resistant composite materials, high-temperature-resistant bonding agents, and the like are required to withstand instantaneous impact such as stress concentration from an external stress. Ideally, the ability of undergoing an elastic deformation like rubber is an important and highly desirable property. The elastic deformation property of a resin is evaluated by measuring the elongation at a break of each matrix resin. The greater the elongation of the matrix resin, the greater the overall strength of a composite material. Thus, the drawbacks of the presence of a reinforcing material such as glass fibers or carbon fibers required for a composite material can be compensated.
For such matrix resins and the like, long-term storage stability, heat resistance and dimensional stability are also important properties. Such resins are also required to undergo less deterioration by light or oxygen in the air. This oxidation resistance is primarily attributable to the structure of each resin. In addition to failing to satisfy the abovementioned demands for mechanical strength, conventional aromatic amine resins involve difficulties in overcoming various drawbacks which are attributed to their structural defects.
The aniline-formaldehyde resin represented by formula (f) above; is converted into a crosslinked structure when the molar ratio of formaldehyde is increased upon condensation thus increasing the degree of condensation and improving its mechanical properties and the like. Accordingly, it is only possible to increase the molecular weight of such a resin to about 600 [Noda, et al., Nippon Kogyo Kagaku Zasshi, 55, 484-487 (1952)].
The present inventors have already discovered novel aromatic amine resins which have improved upon these drawbacks. A patent application covering such aromatic amine resins was filed on Sept. 17, 1987 in Japan (Japanese Patent Application No. 230987/1987). However, since these resins are composed of a secondary amine, it is difficult to achieve isocyanation or maleimidation. They are accompanied by further problems such that when used as a curing agent, these resins must be employed in a relatively large amount and their curing speeds are relatively slow.
On the other hand, thermosetting resins having an imide structure have already found wide-spread industrial utility due to their excellent electrical insulation, heat resistance and dimensional stability of moldings.
However, thermosetting resins which are obtained by separately subjecting aromatic bismaleimides to heat polymerization have drawbacks such as being extremely brittle and having poor flexibility. As a method for improving such drawbacks, it has been attempted to use a thermosetting resin composition composed of an aromatic bismaleimide and an aromatic diamine. For example, a polyaminobismaleimide resin ("Kerimid", trade mark; product of Rhone-Poulanc S.A.) composed of N,N'-4,4'-diphenylmethanebismaleimide and 4,4'-diaminodiphenylmethane has been used widely in impregnating varnishes, laminated boards, moldings, etc. (Japanese Patent Publication No. 23250/1971).
However, such thermosetting resin compositions do not have sufficient heat resistance and are not satisfactory either in impact resistance or flexibility.